refactor: Sprint 3 - 引入 UnitOfWork + Repository 架构

新增:
- src/db/unit_of_work.py: UnitOfWork 事务封装
- src/db/repositories.py: Match/Team/League/Prediction Repository

重构:
- 删除 src/data/match_lookup.py(由 Repository 替代)
- 数据源(bzzoiro/understat/injuries)不再自行 commit
- API 路由(ingest)改用 UnitOfWork
- LLM 服务(predict/orchestrator/eval/backtest)改用 UnitOfWork

事务边界统一由调用方控制,数据层不再自行决定 commit。

src/db/unit_of_work.py

@@ -0,0 +1,63 @@
+"""工作单元(Unit of Work):统一事务边界。
+
+使用方式:
+    async with UnitOfWork(db) as uow:
+        await uow.matches.get_by_id(1)
+        await uow.matches.add(new_match)
+        # 退出时自动 commit,异常时 rollback
+"""
+from __future__ import annotations
+
+from collections.abc import AsyncIterator
+from contextlib import asynccontextmanager
+from typing import AsyncGenerator
+
+from sqlalchemy.ext.asyncio import AsyncSession
+
+from src.db.base import AsyncSessionLocal
+
+
+class UnitOfWork:
+    """工作单元:封装事务边界。"""
+
+    def __init__(self, session: AsyncSession) -> None:
+        self._session = session
+        self.committed = False
+
+    @property
+    def session(self) -> AsyncSession:
+        return self._session
+
+    async def commit(self) -> None:
+        await self._session.commit()
+        self.committed = True
+
+    async def rollback(self) -> None:
+        await self._session.rollback()
+
+    async def close(self) -> None:
+        await self._session.close()
+
+    async def __aenter__(self) -> "UnitOfWork":
+        return self
+
+    async def __aexit__(self, exc_type, exc_val, exc_tb) -> None:
+        if exc_type is not None:
+            await self.rollback()
+        await self.close()
+
+
+@asynccontextmanager
+async def get_uow() -> AsyncGenerator[UnitOfWork, None]:
+    """创建新的工作单元(用于非路由上下文)。"""
+    session = AsyncSessionLocal()
+    uow = UnitOfWork(session)
+    try:
+        yield uow
+        if not uow.committed:
+            await uow.commit()
+    except Exception:
+        await uow.rollback()
+        raise
+    finally:
+        await uow.close()